I guess at this point, why not
Since they're siblings I'm guessing they were also given life by the same man and women, which would make them full siblings. I mean, yes, I'm not surprised at this point, she's been like this for multiple chapters, most chapters if you will. And yes, I understand you want best for your siblings, like how before my brother died, I wanted him to date someone and be happy, and I get this is what she's doing in this is mostly coming from care of her sibling, but she's still stepping over line. But everything on here is fiction so it's doesn't really matter, read what you want, like what you want, don't let anyone bother your day or night. Have a good day.
Author kinda weird
It's like when a straight girl dates a straight or bisexual man to use him as bl material, except, it's his sister. This story is already overly sexualised by the author and just bad, but idk why he had to add another pervert into this to make it worse.